Robby Mook, who was Hillary Clinton’s campaign manager in 2016, said Monday that Russian involvement “could have” cost Mrs. Clinton the election against President Trump.

“I think it could have,” Mr. Mooksaid in response to a question on “CBS This Morning.”

“This is the key: No campaign manager, no candidate should ever sit at a table like this on a news program and say, ‘I don’t know. The Russians might have been the reason we lost the election,’ ” he said.

“It’s not important whether they were or not — what’s important is that it could have been the reason, and that should never happen again,” he said.

Mr. Mook is calling for a bipartisan investigation into possible ties between Russia and Mr. Trump’s team.

White House chief of staff Reince Priebus said on NBC’s “Meet the Press” Sunday that a recent New York Times report that said President Trump’s associates were in frequent communication with Russian officials during the campaign was “total baloney.”